Not applicable.
Not applicable.
The present invention is generally related to optical networks, and in particular provides methods and apparatus for routing wavelength division multiplexed optical signals.
Optical fiber networks are used in a variety of applications, such as optical tele-communication and data transmission systems. Optical fiber networks use optical fibers as transmission lines for carrying light signals. The light signals propagate down the fiber from one location to another, analogous to electrical signals traveling down a wire or cable from one location to another. Optical fibers are used in a variety of applications, such as metro access local loops and xe2x80x9clong haulxe2x80x9d transmission lines. Long haul transmission lines might carry signals between cities or across oceans, for example.
Optical fibers can carry a single channel, or many channels can be multiplexed onto a single fiber. Multiplexing is way of increasing the information-carrying capacity through an optical fiber. There are various ways to multiplex signals on an optical fiber or other type of transmission line, including time division multiplexing and wavelength division multiplexing (xe2x80x9cWDMxe2x80x9d). In a WDM system, a number of wavelength channels are carried on a single optical fiber. A channel is typically defined as a frequency (wavelength) of light that is modulated to carry information.
Optical WDM networks typically allocate a portion of the spectrum about a center frequency of the nominal channel wavelength for signal transmission. For example, channels might be spaced 100 GHz apart with xc2x112.5 GHz on either side of the channel center frequency in a particular system, thus providing the channel with a xe2x80x9cwidthxe2x80x9d of 25 GHz. The remainder of the inter-channel spectra allows for channel separation in order to reduce adjacent channel interference or xe2x80x9ccross-talkxe2x80x9d.
Ideally, the channels could be separated from each other with filters that provided zero transmission loss of the channel and infinite transmission loss of light signals not within the channel. In other words, a filter characteristic (insertion loss versus wavelength) that had a xe2x80x9cflatxe2x80x9d top and xe2x80x9cverticalxe2x80x9d sides. However, actual filters have some insertion loss within the passband (selected channel), including passband ripple, and xe2x80x9cskirtsxe2x80x9d that provide a slope to the out-of-band insertion loss versus wavelength. Furthermore, the filter characteristic can change, with humidity and temperature, for example. Therefore, nominal channel spacing is greater than channel width. Channel spacing of 100 GHz or less is commonly referred to as dense wavelength division multiplexing (xe2x80x9cDWDMxe2x80x9d).
While some applications (i.e. optical transmission systems) might transmit all the optical signals in WDM system from one point to another, other applications might select a channel or number of channels to be handled differently from the rest. For example, an optical signal occupying a channel might be dropped off a multi-channel optical fiber and provided to a user. It is often desirable to add another optical signal or xe2x80x9cpayloadxe2x80x9d occupying the same channel as the dropped signal to utilize the carrying capacity of the optical fiber. This action is known as optical add/drop multiplexing (xe2x80x9cOADMxe2x80x9d). However, OADM present a number of challenges. First, different optical routing paths might create different amounts of loss for different channels. In order to keep the signal strength of all channels about the same, for broadband amplification or other signal processing, for example, differential attenuation or amplification (i.e. varying the signal strength of one channel with respect to another) might be performed.
Another design challenge is to provide add/drop filters with sufficient in-band flatness and insertion loss while providing high out-of-band rejection. While dielectric thin-film filters provide relatively good rejection and passband flatness, optical telecommunication system requirements can be difficult to achieve with a single filter. A dielectric thin-film filter typically uses alternating layers of high and low (relative to each other) dielectric material, such as metal oxides, of a selected thickness, such as a quarter wavelength or a half wavelength thickness. Variations in the layer thickness(es), composition of the dielectric material, design compromises (such as passband width) and other variables result in an actual filter that does not equal the performance of the designed (hypothetically perfectly fabricated) filter, which again cannot meet the ideal square-top filter shape. Thus, both insertion loss and out-of-band rejection of actual filters are less than ideal.
Insertion loss is important for at least two reasons. First, it is desirable to pass (drop) the desired optical signal or channel(s) through the filter with minimal loss. Second, a particular type of cross talk can occur during OADM, namely the residual signal from the dropped channel can interfere with the added channel, which is at the same wavelength. That is, some of the dropped channel remains on the expressed signal.
Thus, it is desirable to provide optical routing networks that are loss balanced and provide low residual signals for use in OADM WDM systems.
The present invention provides an optical routing array with low insertion loss for re-inserted channels and a bypass path that has similar insertion loss. In one embodiment, the optical routing array has four switching paths and one bypass path, but applies to more or fewer paths. Each of four switching paths is routed from a common optical input and to an array optical output with a similar number of transmission and reflection losses. The bypass path carries bypass channels that are not routed through one of the switching paths in the array, but are coupled from the input to the output. The bypass path loss is within about 3 dB of each of the four switching paths when in the re-insertion configuration. In one embodiment, each of the four optical paths carries an optical channel of a WDM optical signal. In another embodiment, at least one of the optical paths carries a number of adjacent optical channels of a WDM optical signal. In a further embodiment, each of the four optical paths carries a number of adjacent optical channels of a WDM optical signal. In still a further embodiment, each of the four optical paths carries the same number, N, of adjacent optical channels of a WDM optical signal, where the total number of optical channels carried on the WDM system is the number of paths (i.e. four) times N.
In another embodiment, a stub filter is added to an optical routing array to reduce the residual signals reflected from the inputs of an add/drop nodes. An add/drop node typically has two wavelength-selective filters reducing residual signal between the common input and the array output when a bypass path between the input side and the output side of the array is provided. The addition of the filter stub provides a third wavelength-selective filter to reduce the strength of residual signals. In one embodiment, the filters in the routing node (i.e. switchable path) and the filter stub are bandpass filters. In another embodiment, the input and output filters of the routing node are bandpass filters and the stub filter is a band-edge filter. Each filter provides between about 12-17.5 dB of suppression. Generally, each of the three filters do not have the same amount of suppression, hence the cumulative suppression can vary. The three wavelength-selective filters provide a cumulative suppression of the residual signal(s) on the array output of greater than 36 dB. In another embodiment the cumulative suppression sums to about 45 dB, and in another embodiment up to about 53 dB.